Asymptomatic mycobacterial infections are characterized by the presence of very few bacilli which are difficult to find, may not be acid-fast, and may not be culturable. These bacilli have been proposed to persist within closed airways of the mammalian lung under what have been described as microaerophilic or anaerobic conditions. We have begun characterizing the biochemical changes which accompany the adaptation to microaerophilic survival and replication. We have shown that the mycobacterial a-crystallin protein is specifically expressed during stationary-phase and under low-oxygen conditions in virulent mycobacterial strains. This protein was overexpressed and purified and shown to have ATP-independent chaperone function and to provide protection from autolysis to aging bacteria. Targeted gene replacement was used to create a genetic knockout of the alpha crystallin protein in virulent MTB and the resulting strain shows dramatically decreased abilities in growth and replication in macrophage models of infection.